The present invention relates to a belt driving device for revolving an endless belt entrained about a plurality of entraining rollers.
Among electrophotographic image forming apparatuses for example, there exists an apparatus of the type which includes an endless belt such as an intermediate transfer belt, a secondary transfer belt, and a fixing belt. One known belt driving device for revolving such an endless belt includes a plurality of entraining rollers positioned parallel with each other for entraining the endless belt thereabout and is configured to revolve the endless belt by actuating a driving roller of the entraining rollers. Such a belt driving device involves a problem that the endless belt meanders because of variations in the outer diameters of the entraining rollers due to manufacturing errors, parallelism disorders between the entraining rollers due to mounting errors, or the like. In an image forming apparatus in particular, it is critical to limit the meandering of the endless belt as much as possible because the meandering of the endless belt affects the image quality adversely.
One known technique for limiting the meandering of an endless belt is based on an electric control such as to detect the amount of deviation of the endless belt and move a meandering correction roller in accordance with a detection signal. However, such an electric control based technique, however, requires a higher parts count due to the provisions of sensors, electric circuits, driving motor and the like, which raises a problem that the structure becomes complicated while the cost becomes higher.
A technique developed in view of these problems is known which limits the meandering of an endless belt by adjusting the tension of the endless belt by means of a mechanical system without using any sensor (see Japanese Patent Laid-Open Publication No. 2005-162466 for example). According to this conventional technique described in Japanese Patent Laid-Open Publication No. 2005-162466, a tension roller is rotatably supported by an arm member. The arm member is pivotable in the running direction of the endless belt and is biased by a spring toward the downstream side in the running direction. On opposite sides of the tension roller there are provided belt contact portions. When the endless belt meanders, the endless belt is brought into contact with the belt contact portions. As the width of contact between the endless belt and each belt contact portion increases, frictional force exerted on the belt contact portion increases. It is intended that such frictional force and the biasing force of the spring should turn the tension roller to increase the tension of the endless belt.
With the conventional technique described in Japanese Patent Laid-Open Publication No. 2005-162466, however, the pivoting direction of the arm member for increasing the tension of the endless belt is the same as the running direction of the endless belt. This might make this technique unable to adjust the tension of the endless belt because increasing frictional force exerted on the belt contact portions with increasing amount of deviation of the endless belt causes the arm member to go beyond the peak and fall down on the downstream side in the running direction of the endless belt by the force of the endless belt running, with the result that the tension imparted by the tension roller to the endless belt becomes extremely weak. In order to prevent the arm member from going beyond the peak, various settings including setting of spring strength, setting of the length and mass of the arm member and like settings have to be made with difficulty. Therefore, it is difficult to correct the meandering of the endless belt stably.
Where the above-described conventional technique is applied to a belt driving device for use with an endless belt having a stretch property such as a secondary transfer belt in spite of the nature of the stretchable endless belt such that the endless belt is likely to deviate toward a higher tension side, the tension on the side toward which the endless belt deviates is increased, which causes the endless belt to meander increasingly vigorously.
A feature of the present invention is to provide a belt driving device which is capable of stably correcting the meandering of an endless belt by means of a simple mechanism.